In general, on a sidewall of a pneumatic tire, a variety of characters, numbers and graphics (hereinafter, referred to as a display portion) are provided in order to meet legal criteria of the respective countries around the world.
When the display portion is provided, from a viewpoint of improving an exterior appearance of the product, for example, as shown in FIG. 1, display portions 103 are sometimes provided on an area of a decorative portion 101 in which ring-shaped longitudinal ridges having cross sections composed of crests and roots are applied, that is, so-called serrations are cut.
The display portions 103 are higher than the decorative portion 101 and have a shape in which each upper surface 105 has a large surface area when being enlarged as shown in FIG. 2.
As apparent from FIG. 2, rigidity of the display portions 103 provided on the sidewall becomes larger as compared with that of the decorative portion 101 in terms of a relationship in cross-sectional area therebetween. Accordingly, a rigidity difference occurs between each display portion 103 and the decorative portion 101 through a root groove 107.
The rigidity difference becomes a factor to cause a stress concentration in the root groove 107 as a boundary between the display portion 103 and the decorative portion 101 due to an influence of a load applied repeatedly when the tire runs. In particular, in a region of the decorative portion 101 sandwiched between the display portion 103 and the display portion 103, which have high rigidity, the rigidity difference becomes noticeable.
When the stress concentration occurs repeatedly in the root groove 107, such an occurrence is prone to lead to a crack due to flex fatigue of the root groove 107, causing a problem that durability of a region of the display portions is greatly affected.
Note that, as a document of the prior art related to this application, the following is given.
Japanese Patent Laid-Open No. H6-340208 (published in 1994)